1. Field of the Invention
The subject invention relates to the processing of composite video signals, including chrominance signals, to the correction of time base errors in such signals, and to the playback of recorded video signals.
2. Description of the Prior Art
Despite the remarkable progress made in recent years in the field of color video signal recording and playback, there persists a need for improved equipment which will prevent color degradation or loss that occurs with existing apparatus in response to frequency variations in available alternating-current power supplies.
More specifically, a well-known type of video tape recorder records the video information in the form of separate luminance and chrominance signals. For the purpose of recording, the chrominance signals are relocated on a carrier frequency which is lower than the standard chrominance subcarrier frequency. Upon playback, the sidebands of that lower recording carrier are beat against a signal that is affected by the same time base errors as the played back chrominance signal, but that has a frequency equal to the sum of the standard chrominance subcarrier frequency and the special recording carrier frequency. In this manner, it is possible to obtain a difference signal which contains the chrominance information in sidebands of a carrier having the standard subcarrier frequency and which is essentially free from time base errors.
Prior art in this area uses an adjustable color lock control signal to enable the continued elimination of time base errors in the above mentioned manner. In general, the system just described is workable where the phase difference between the signals that are beat as mentioned above to relocate the chrominance sidebands about a standard subcarrier frequency, remain within about one percent tolerance. Beyond that, color loss occurs with the prior-art approach just described.
In an attempt to cope with this kind of problem in practical applications, some prior-art equipment has an externally accessible, manually adjustable color lock control potentiometer which, upon manual adjustment, will permit the voltage controlled oscillator to follow occurring time base errors.
Unfortunately, manual color restoration is impossible or at least impractical in many situations. For instance, if the video playback equipment is to be employed in the context of an airborne video passenger entertainment system, existing safety regulations generally require a packaging of the equipment which inhibits provision and practical use of manually adjustable color lock controls.
Moreover, the frequency of the power generated aboard aircraft and other vehicles is seldom as accurate. as the frequency of the power available from public utility systems and similar entities. For instance, frequency variations within a two percent positive and negative swing relative to a nominal frequency are not rare aboard aircraft and in other situations where power is generated locally. This, of course, means among other things that the speed of a video head scan drum will vary over a four percent span with a corresponding introduction of time base errors into the reproduced video signal, resulting in frequent loss of color.
The more sophisticated prior-art proposals for preserving color in reproduced video signals typically run into problems of complexity and bulkiness which render them unsuitable for use aboard aircraft or in relatively low-cost and light-weight equipment.
The control of chrominance signal processing in response to fluctuations in a video synchronization signal, such as the horizontal sync signal, is not new as such. For instance, a well-known video tape recorder and playback machine employs the horizontal sync signal to distinguish between the regular playback mode in which recorded color video information is reproduced for motion picture type display and the still mode in which selected portions of the recorded color video information are repeatedly reproduced for still picture display.
However, there is a lack of suitable equipment providing compensation of time-base errors of the chrominance component of a composite color video signal induced by larger frequency variations in a local power supply.